The simplest way to get this right is to follow certain structures for these question types.
IF PAST, THEN PAST
IF PAST, THEN WOULD
IF PAST PERFECT, THEN WOULD HAVE
Stacy would have gone to the party if she knew about it - This sentence is wrong because of the usage 'would have' along with a simple past 'she knew'.
The error can be corrected by using a past perfect in the if clause.
Correct - Stacy would have gone to the party if she had known about it
Hope that makes sense.
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